Neurons use nervous impulses to communicate with each other and to stimulate muscles and glands. These impulses, caused by the movement of ions across the plasma membrane, are characterized by changes in voltages across the membrane (membrane potentials measured in mV). Locally induced graded potentials may trigger action potentials, which are propagated over long distances.
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Neurons, electrically excitable cells, respond to chemical and mechanical stimuli by producing small changes in the resting membrane potential (measured in mV) called graded potentials. Graded potentials, if large enough, may produce an action potential. Action potentials are generated when the resting membrane potential (usually -70 mV to -55 mV) at the axon hillock is depolarized above threshold (usually-55 mV) by depolarizing graded potentials.
An action potential is a large change in membrane potential. It is used to communicate over large distances. Action potentials are generated when the resting membrane potential (usually -70 mV to -55 mV) at the trigger zone experiences a large enough graded depolarization to exceed threshold voltage (usually -55 mV).
When threshold voltage is reached, voltage gated sodium channels open allowing positively charged sodium ions to enter the neuron, causing the inside of the cell to become more positive. During the depolarization phase of an action potential so many sodium ions diffuse into the neuron that the membrane potential actually reverses charge (a spike to +30 mV).
Depolarization is quickly followed by repolarization when voltage gated potassium ion channels open allowing diffusion of positively charged potassium ions out of the cell. Repolarization causes the inside of the cell to become more negative again, eventually restoring the resting membrane potential of -70mV.
During most of an action potential, the neuron is in a refractory period. Another stimulus during the refractory period generally does not cause a new action potential. Once the axon has repolarized to - or near to - the resting membrane potential, a new action potential can be generated.
During an action potential, depolarization of a section of the plasma membrane causes the adjacent section of the axon to also depolarize above threshold and generate a new action potential. In this way, the action potential is propagated down the axon. The amplitude (height of voltage spike) during depolarization remains constant as the action potential travels down the length of the axon.
Voltage changes in membrane potentials can be measured using patch clamp equipment in constant-current mode. Patch clamp electrodes are placed at different positions along the neuron. Special lighting and microscopy is required to visualize non-stained neurons.
Changes in the membrane potential can be observed by using a voltage-sensitive fluorescent dye. The change in membrane potential can be seen as color changes moving along the neuron.
